We discuss the existence of large, complex merged
interaction regions (MIRs) in the solar wind near Earth.MIRs can have configurations that cause more
prolonged geomagnetic effects than a single flow structure.A MIR or successive MIRs can produce
relatively long lasting Forbush decreases at 1 AU.We illustrate MIRs at 1 AU with two examples (MIR-1 and MIR-2)
seen by WIND and ACE in the interval from March 18 through March 29, 2002.We determined the probable structure and
origin of each in terms of interacting flows and shocks using in situ and solar
observations, but we emphasize that there are uncertainties that cannot be
resolved with these data alone.The
MIRs were relatively large structures with radial extent » 2/3
and 3/4, respectively.MIR-1 was formed
by interactions related to at least two complex ejecta, a magnetic cloud, and
two shocks.MIR-2 was related to a
corotating stream, the heliospheric plasma sheet (HPS), two complex ejecta, a
magnetic cloud and at least two shocks.A MIR can evolve significantly while it moves to 1 AU, and memory of the
conditions near the Sun is lost in the process.Thus, one cannot unambiguously determine the structure of a MIR
and the manner in which it formed using observations from a single spacecraft
at 1 AU.The magnetic field strength profiles
in MIRs are not correlated with the speed and density profiles, so that one
cannot infer the magnetic field strength in MIRs from remote sensing
observation that give density and speed information.It will be possible to better understand the dynamical processes
leading tot he formation of MIRs with remote sensing observations, but they
cannot measure the magnetic fields in MIRs.